This invention relates generally to memories that use phase-change materials.
Phase-change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure and the crystalline state involves a more ordered atomic structure. Generally, any phase-change material may be utilized; however, in some embodiments, thin-film chalcogenide alloy materials may be particularly suitable.
The phase-change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter or vice versa. In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states.
In some situations, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance determined states may be possible allowing the storage of multiple bits of data in a single cell.
A variety of phase-change alloys are known. Generally, chalcogenide alloys contain one or more elements from column VI of the periodic table. One particularly suitable group of alloys are GeSbTe alloys.
A phase-change material may be formed within a passage or pore defined through a dielectric material. The phase-change material may be coupled to contacts on either end of the passage.
The phase-change may be induced by heating the phase-change material. In some embodiments of phase-change memories, a current is applied through a lower electrode that has sufficient resistivity or other characteristics to heat the phase-change material and to induce the appropriate phase change. In some embodiments, the lower electrode may produce temperatures on the order of 600° C.
One problem with existing electrode arrangements is that the higher the temperature, the lower the resistivity of the material. Thus, as the lower electrode is heating up in order to induce the phase change, it progressively becomes less resistive, thereby decreasing the amount of heat that is generated.
Thus, there is a need for a controllable way to provide sufficient resistance proximate to the phase-change material even at elevated temperatures.